In the past two months several cannabis farms, some with up to 700 plants, have been found during heavy frosts. Conditions needed to grow the plants are so hot that ice melts from roofs.

In Northern Ireland the PSNI has uncovered more than 100 "dope" factories, with £15m of drugs seized. A total of 72 people from southeast Asia have so far been charged by the PSNI, with detectives investigating possible connections to Chinese and Vietnamese gangs exporting to Britain.

In the south, cannabis factories are being uncovered on a weekly basis by gardai in suburban estates, on remote farms and in warehouses.

The security forces can also use thermal imaging equipment on helicopters and patrol cars to detect unusually warm buildings.

High electricity consumption can betray the presence of a cannabis factory,